VARIOUS METHODS OF HEATING DESCRIBED IN DETAIL. 193 



has been ascertained that air heated to a temperature of 300 de- 

 grees, becomes so deprived of its organic matter, and otherwise 

 changed in its properties, as to be unfit for the sustenance of 

 either animal or vegetable life, in a state of healthy and vigorous 

 development, for any length of time ; and hence the admission 

 of a current of highly heated air into a dwelling room, or into 

 a well glazed hot-house, if no means are taken to restore its 

 original properties, must, in a short time, become sensibly in- 

 jurious to the animals and vegetables that are compelled to 

 breathe it. 



And this we find to be practically the case. Every gardener, 

 on entering a hot-house so heated, is immediately sensible of 

 the presence of contaminating gases in the atmosphere, whether 

 arising from the combustion of fuel, or otherwise, and he is too 

 well acquainted with its effects on vegetative beings to allow 

 his tender plants to absorb it ; hence he takes immediate meas- 

 ures of modifying what he cannot possibly prevent. It can 

 scarcely be doubted, that a vast amount of sickness and diseases 

 of the respiratory organs is, in a great measure, attributable to 

 the same circumstance, especially in people of sedentary habits, 

 who confine themselves to close chambers, warmed by currents 

 of hot air, or highly heated stoves. The latter, in this respect, 

 is probably worse than the former ; for, in the one, the supply of 

 air to be heated is drawn from the external atmosphere, and, 

 consequently, is less likely to contaminate the air of the room, 

 although, when conducted into the room at high temperatures, 

 the atmosphere of the latter, without egress as well as ingress 

 of air, must ultimately become so. In the case of stoves, how- 

 ever, it is different, for by them the same atmosphere is heated 

 over and over again, by convection. The particles of air in 

 contact with the stove first become heated, these expand with 

 the heat, and, consequently, becoming lighter, rise, and the 

 colder particles supply their place, which also expand, rise, and 

 are in their turn replaced by others. Here the supply of air to 

 be warmed is drawn directly from the room itself; thus com- 

 pelling the inmates to inhale the same contaminated atmos- 

 phere for days together, without mixture or admission of fresh 

 air, except the small portion that finds an unwelcome entrance 



